Milling apparatus for implanting a joint prosthesis

ABSTRACT

A novel and improved elbow prosthesis and method of implanting same including a prosthesis system that has a retaining system. The joint prosthesis system can include an implant, and an alignment guide. The alignment guide can include a post member extending through a central axis of rotation in a prosthesis and a drill guide member having a first limiting member for limiting articular surface removal from a joint. The first limiting member can include a recessed guide for positioning the implant.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.17/030,276, filed Sep. 23, 2020, which is a divisional of U.S. patentapplication Ser. No. 15/711,899, filed Sep. 21, 2017, now U.S. Pat. No.10,799,253, which is a continuation of U.S. application Ser. No.13/193,196, filed Jul. 28, 2011, now U.S. Pat. No. 9,782,184, which is adivisional of U.S. application Ser. No. 12/011,337, filed Jan. 25, 2008,now U.S. Pat. No. 8,012,214, which is a continuation-in-part of U.S.patent application Ser. No. 11/237,171, filed Sep. 27, 2005, now U.S.Pat. No. 8,034,113; each of these applications is incorporated byreferenced herein in its entirety.

BACKGROUND

The article of manufacture and method relate broadly to a jointprosthesis and method of implanting same, and more particularly to amammalian joint prosthesis and novel and improved method of implantingsame.

The majority of mammalian joints are hinge-type synovial joints formedwhere a distal end of a bone articulates with a proximal end of anopposite or complementary bone. Joint dysplasia is a common debilitatingcondition that affects mammals and more specifically the canine elbowjoint. The current surgical techniques result in an unacceptable failurerate of the implant due to the technical difficulties associated withthe implantation procedure as well as excessive post-surgical physicaltherapy needs as a result of the invasiveness of the procedure and theabundance of soft tissue damage.

There is therefore a need for a novel and improved joint arthroplastythat involves a minimally invasive surgical technique with a novelimplant. The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods which aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above described problems havebeen reduced or eliminated, while further embodiments are directed toother improvements.

SUMMARY

The embodiments and methods set forth are exemplary and not for purposesof limitation. The present embodiments and methods are designed toprovide a novel and improved elbow joint prosthesis and method ofimplanting same incorporating a first member having a first bonefixation surface and a first articulating surface, a complementarysecond member having a second bone fixation surface and a secondarticulating surface the second articulating surface portion havingintersecting concave and convex surfaces defining alternate upwardly anddownwardly curved projections as well as an opposite second bonefixation portion, the first and second bone fixation surfaces beingdisposed opposite to the first and second articulating surfaces, and aset plate member adapted to receive the first and second members.

The first and second members form an articulating prosthetic jointimplant. The implant utilizes unique bone-stabilizing pegs as well asbone-receiving beads promoting bone ingrowth and reducing asepticloosening. The anatomical duplication of the joint preserves flexion andextension while reducing excessive pulling of ligaments. A novel setplate member releasably links the first and second members and aids inpositioning of the implant, forming an aggregate implant.

Methods are also provided for a novel and improved joint arthroplasty.One such method, offered by way of example but not limitation, ofimplanting an endoprosthesis comprises the steps of exposing a medial orlateral joint of a subject, implanting a pin member through a centralaxis of rotation of a joint, drilling prosthesis post cavities in thejoint, milling articular surfaces and press-fitting the prosthesis. Themedial approach in elbow joint arthroplasty, which is usually the areamost affected by elbow dysplasia is proposed but other approaches suchas lateral may be used as well. This will result in a lower failure rateof the implant due to superior biomechanics of the implant, a lowerdegree of invasion of the joint capsule and ligamentous structure whilereducing periarticular scarring. Milling arthroplasty results in lessstructural damage to the joint, provides good trabecular structure tosupport the implant without subsidence, low infection rates and littlebleeding. The current implant may be inserted without disarticulatingthe joint thereby enabling an earlier return to weight bearing andwalking while providing for a minimally invasive technique. Theimplantation of all members of a total joint prosthesis with only oneimplantation step is novel and reduces trauma to the subject.

The foregoing examples of the related art and limitations relatedtherewith are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those skilled inthe art upon a reading of the Specification and study of the Drawings.In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to theDrawings and by study of the following Description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an embodiment of a joint prosthesisincluding a set plate;

FIG. 2 is a side view of the implant of FIG. 1 including the humerus,radius and ulna;

FIG. 3 is a side view of the implant of FIG. 1 without a set plate;

FIG. 4 is a top plan view of the implant as shown in FIG. 1 ;

FIG. 5 is a bottom plan view of the implant as shown in FIG. 1 ;

FIG. 6 is a side view of the implant of FIG. 1 including the set plate;

FIG. 7 is a perspective view of the implant shown in FIG. 1 ;

FIG. 8 is a perspective view of a set plate;

FIG. 9 is a perspective view of a humeral component;

FIG. 10 is a bottom plan view of the humeral component of FIG. 9 ;

FIG. 11 is a perspective view of an ulnar component;

FIG. 12A-12D are multiple views of the set plate shown in FIG. 1 ;

FIG. 13 is a cross-sectional view about lines 13-13 of the implant shownin FIG. 11 ;

FIG. 14 is a cross-sectional view about lines 14-14 of the implant shownin FIG. 7 ;

FIG. 15 is a perspective view of a positioning system;

FIG. 16 is a perspective view of the positioning system shown in FIG. 15;

FIG. 17 is an exploded perspective view of an alignment plate, a centerof rotation post and an aggregate prosthesis;

FIG. 18 is a top plan view of the assembly of the alignment plate ofFIG. 17 including the humerus, radius and ulna;

FIG. 19 is a side view of the alignment plate of FIG. 17 ;

FIG. 20 is a bottom view of an alignment plate;

FIG. 21 is a perspective view of an alignment plate;

FIG. 22 is a perspective view of an alignment plate, center of rotationpost and drill guide;

FIG. 23 is an exploded view of an alignment plate, center of rotationpost and a drill guide;

FIG. 24 is a side view of a drill guide;

FIG. 25 is a bottom view of a drill guide;

FIG. 26 is a perspective view of a drill guide;

FIG. 27 is a top plan view of a retraction plate shown in FIG. 22 ;

FIG. 28 is a perspective view of a retraction plate;

FIG. 29 is a perspective view of a milling arm, alignment plate andcenter of rotation post;

FIG. 30 is an exploded view of a milling arm, an alignment guide and acenter of rotation post;

FIG. 31 is a perspective, exploded view of a milling arm;

FIG. 32 is a top plan view of a milling arm;

FIG. 33 is a side view of the milling arm of FIG. 30 ;

FIG. 34 is a bottom view of the milling arm of FIG. 30 ;

FIG. 35 is a perspective view of an end of the milling arm of FIG. 30 ;and

FIG. 36 is a perspective view of a second end of the milling arm of FIG.30 .

Exemplary embodiments are illustrated in referenced figures. of thedrawings. It is intended by the embodiments and figures disclosed hereinare to be considered illustrative rather than limiting.

DETAILED DESCRIPTION

In the embodiments shown in FIGS. 1-14 , there is provided an implant11′ with a humeral condylar component 13′ and a radioulnar component29′. The humeral component 13′ includes a first articulating surfaceportion 17′ of substantially saddle-shaped configuration, as shown inFIGS. 1, 9 and 10 , and an opposite first bone fixation portion 14′, asshown in FIGS. 1, 4 and 9 . Geometrically, the saddle-shapedconfiguration of the first articulating surface portion 17′ is broadlyin the form of a concave configuration along planes parallel to itsmajor axis and generally convex in planes transverse to the major axis,the degree of convexity lessening or flattening out, toward the centerof the major axis. The humeral condylar component 13′ is generallyhalf-moon shaped, having an outer convex articulating surface 17′ andinner concave surface, and made of cobalt-chrome molybdenum (Co-CrMb),titanium, Ti-alloy or ceramic but may also be made of other materials.The first articulating surface 17′ as shown in FIG. 10 has alongitudinally extending angular groove 15′ and simulates orapproximates the natural shape of a trochlea humerus which on a caninefor example, is a medially located, arcuate pulley-shaped member. Thegroove 15′ extends diagonally across the surface and extends at an acuteangle to an imaginary plane through a major axis of the firstarticulating surface portion 17′. The first bone fixation portion 14′ ofthe humeral component 13′ has a concave, semi-circular surface 21′. Theconcave surface 21′ is complementary to the first articulating surface17′ and includes transversely extending cylindrical retaining members orprotuberances 40′, 40″. The retaining members 40′, 40″ may be hollow orhave shallow openings 41′, 41″ at one end with the open end extending upto outer peripheral edges 16′ of the first bone fixation portion 14′.The openings 41′, 41″ are designed to receive a set plate 47′ which willbe discussed in more detail at a later point. The retaining members 40′,40″ typically are evenly spaced from the ends and extend transversely toa major axis of the humeral component 13′. The retaining members 40′,40″ may extend the width of the component 13′ and in this embodiment donot extend beyond the outer peripheral edge 16′ of the humeral component13′. Alternatively, the retaining members 40′, 40″ could extend beyondthe outer edges of the component or could end short of the outer edgesof the component. The first bone fixation portion 14′ may also includeporous members, such as, PCA beads 27′ which also promote bone growth.The PCA beads are manufactured by Bio-Vac, Inc., of Michigan, USA. Otherpossible fixation members include hydroxyl apatite (HA) coating,titanium plasma spray coating or Resorbably Blast Media Coating to namea few. Bony fixation of prosthetic implants is encouraged with surfaceextensions, such as, the retaining members 40′, 40″ which form a primaryfixation for the implant. Beaded porous members provide secondaryfixation allowing for bony ingrowth. A proximal portion 20′ of thehumeral component 13′ which is the first bone fixation portion 14′contacts a distal surface 28′ of the humerus 12′ providing for aninterference fit between the bone fixation portion and the humerus 12′,as shown in FIG. 2

The radioulnar component 29′ has opposing surfaces including a secondarticulating surface portion 31′ and a second bone fixation portion 33′.The radioulnar component 29′ is half-moon shaped and in the embodimentshown is slightly tapered at a posterior end 30′. Other forms of thecomponent may also be used without a slightly tapered end as thecomponent is designed to approximate the joint component of a particularspecies. The second articulating surface portion 31′ has a saddle-shapedor concave configuration that faces cranially. The articulating surfaceportion 31′ contains a medial ridge member 37′ having intersectingconvex and concave surfaces defining alternate upwardly and downwardlycurved projections. The ridge member 37′, in the embodiment shown,simulates a trochlear ridge but may also be designed to simulate theapproximate anatomy of the joint such as an intercondylar eminence,articulating surface of the talus or trochlea of the human, and iscomplementary to the groove 15′ of the first articulating surfaceportion of the corresponding component 13′. The ridge 37′ as shown inFIG. 11 , extends diagonally across the concave surface at asubstantially mid-level portion between the concave and convex surfacesand extends at an acute angle to an imaginary line through a major axisof the second articulating surface portion 31′. The extension of theridge 37′ approximates the natural helical shape of a trochlear notch ina canine.

As shown in FIG. 2 , the second articulating surface portion 31′ of theradioulnar component 29′ slides or pivots about a major axis (i.e. thecentral axis of rotation of the joint) in relation to the firstarticulating surface portion of the humeral component 13′, forming anarticulating system. The radioulnar component 29′ as shown in FIGS. 3,11 and 13 is made of two pieces, namely, the second articulating surfaceportion 31′ and the second bone fixation portion 33′. The secondarticulating surface portion 31′ is made of ultra-high molecular weightpolyethylene but may also be made of other materials such as PEEK andPEKK. The second bone fixation portion 33′ may be composed of castcobalt chrome molybdenum, titanium or ceramic, or porous tantalum aswell as other materials. This allows the articulating surfaces of thehumeral and radioulnar components 17′ and 31′ to have metal-on-plasticcontact. Other combinations may be used without departing from theintent of providing a smooth, articulating surface.

The second bone fixation portion 33′ of the radioulnar member 29′contains at least one retaining member 23′, but in the case at hand hastwo retaining members 23′, 23″, with hollow openings 22′, 22″ to aid inimplant positioning. As with the transversely extending retainingmembers 40′, 40″ of the humeral component 13′, the retaining members23′, 23″ of the radioulnar component 29′ also may be cylindrical andtransversely extending along a major axis of the radioulnar component29′. Further, the retaining members 23′, 23″ in this embodiment as shownin FIGS. 11 and 13 do not extend beyond an outer peripheral edge 42′ ofthe second articulating surface portion 31′ of the radioulnar component29′. This is by way of example, but the retaining members may alsoextend beyond the outer peripheral edges of the radioulnar component orend short of the outer peripheral edges as well. As with the humeralcomponent 13′, the second bone fixation portion 33′ of the radioulnarcomponent 29′ may also integrate porous beads 27′ to promote boneingrowth as shown in FIGS. 4 and 5 . A distal portion 45′ of theradioulnar component 29′ which is the second bone fixation portion 33′,contacts proximal surfaces of the ulna 51′ and radius 53′ providing foran interference fit between the second bone fixation portion 33′ and theradius and ulna.

In one embodiment, the groove 15′ and ridge member 37′ are not centeredbut the complementary components are longitudinally extending andintersect a major axis only at the center as discussed previously,requiring a different joint prosthesis for the right and left joints. Itwill be evident that in another embodiment, the prosthesis, includinghumeral and radioulnar components, is isometric, and can be used for aright or left joint arthroplasty with the complementary componentsextending longitudinally along a centered vertical plane.

The articulating surfaces of the humeral and radioulnar components arepolished to a smooth finish promoting unencumbered articulation betweenthe two surfaces. The bone fixation portions of the humeral andradioulnar components contain the retaining members 23′, 23″ and 40′,40″ on their surfaces to provide initial stabilization to promote boneingrowth. The humeral and radioulnar components 13′ and 29′, typicallyhave a specific thickness when combined of 8 mm, however this couldrange from 2 mm to 40 mm depending upon the size of the joint. Thehumeral and radioulnar components are releasably linked together with analigning or retaining piece or set plate 47′ as shown in FIGS. 8 and12A-12D that aids in positioning of the implant 11′ within the jointcavity 48′, as shown in FIG. 2 , and is removed once the implant issecurely in place. The combination of the joint prosthesis 11′ and theset plate 47′ forms an aggregate joint prosthesis 10′. The implant setplate 47′, for example, has at least two arms extending outwardly and inthis embodiment has four arms extending outwardly. The set plate 47′ isinserted simultaneously with the components into the joint cavity, to bediscussed in greater detail, and serves multiple functions; due to thecomplexity of articular surfaces of a mammal in general, it is necessarythat when placed in the subject, both components be oriented at theproper depth and in the proper state of articulation as is defined bythe surgical procedure and specifically the drilling and millingprocess, as well as guaranteeing proper alignment between articularsurfaces of both components, in this case the humeral component 13′ andthe radioulnar component 29′, eliminating the possibility of jointcompartment loading with misaligned implants primarily caused bysurgical errors in surface preparation and implant insertion. The setplate 47′ provides a one-step method of joint insertion, reducingsurgical error and guesswork while minimizing trauma to the subjectjoint. Prior art prosthetics in general may fail based on improperloading of the joint surfaces due to misalignment of multi-componentprosthetics. Typically, a joint prosthesis is implanted using multiplesteps including for example, implantation of a humeral componentfollowed by implantation of a radioulnar component in a canine. Forexample but not by way of limitation, the canine elbow is typicallyaligned at 90° flexion for surgical purposes, as shown in FIG. 2 . The90° flexion provides the maximum overlap of articular surfaces of thejoint while allowing for the minimum amount of joint preparation for allpertinent surfaces simultaneously. The implants, to function correctlytogether, should both be at their respective 90° of flexion and insertedto a proper depth.

The set plate 47′ has four end members 57′, 59′, 61′ and 63′, as shownin FIG. 12D, including posts of variable size and shape 49′, 50′, 52′and 54′ that releasably link the humeral component 13′ and theradioulnar component 29′, as shown in FIGS. 1, 4, 6, 7, and 13 . The setplate 47′ may be made of cobalt chrome or plastic and the posteriorulnar post 54′ on the retainer 47′ is slightly larger which compensatesfor the tapering in the posterior end 30′ of the radioulnar component29′ and assures that the implants cannot go in crooked or at an angle tothe sagittal plane that exists at the elbow at the point of intersectionbetween the center line of the humerus and the center line of theradioulnar component. The retainer 47′ also provides a surface forimpaction of the implant upon which one can hammer or press to assuremaximum insertion of the aggregate implant 10′ into the joint cavity.The retainer 47′ is then removed from the joint cavity while the implant11′ remains securely within the joint cavity. Due to the nature of theimplant, the radioulnar component 29′ relies heavily upon the press-fitnature of the component to insure stability. The humeral component 13′is captured between the medial and lateral epicondyles preventingmovement laterally on a frontal or transverse plane.

As embodied and broadly described herein, the elbow arthroplasty of thepresent embodiment includes a humeral component 13′ and a complementaryradioulnar component 29′ as well as the set plate 47′. The aggregateprosthesis 10′ is implanted in one stage as opposed to separate stagesas previously conducted in the prior art.

The implant 11′ is lined up with the implant retaining plate 47′ inplace, all four post members 49′, 50′, 52′ and 54′ lining up with thefour horizontal retaining members 40′, 40″, and 23′, 23″ located on thefirst bone fixation portion 14′ of the humeral component 13′ and thesecond bone fixation portion 33′ of the radioulnar component 29′. Thisallows the aggregate implant 10′ to be inserted where the articulatingsurfaces have been removed. Using a hammer or press device, not shown,the aggregate implant 10′ will be tapped or pressed into place withinthe joint cavity. The press-fit nature of the implant allows for primaryfixation of the prosthesis. With the transverse retaining members, theimplant may not rotate on a sagittal plane while inside the elbow. Thetransverse retaining members also prevent the implant from sliding sideto side based on a press-fit of the joint and stabilize the implant sothat bony ingrowth into porous surfaces may occur, which is thesecondary fixation that occurs.

The aggregate implant 10′ is placed above the cavity created by thedrilling and milling process, to be discussed at a later point, and isimpacted or pressed into the cavity until it reaches the proper andpre-defined depth. As a result of the accuracy and reproducibility ofthe drilling and milling process, there is almost no distance betweenthe aggregate implant 10′ and the bone. Optimally, the implant is setwithin 1 mm of the bone. If there is more than 1 mm. of space betweenthe implant 11′ and the bone, there is increased potential for poor boneingrowth. Cementless fixation is utilized in our method but is set forthas an example, not as a limitation. Once the aggregate implant 10′ is inplace, the set plate 47′ is removed manually. The medial condylar crown,including the attached ligaments and muscles, is reattached, not shown,using a 3.5 mm cancellous screw, not shown. The cancellous screw ismanufactured by Veterinary Orthopedic Implant, Synthes or New GenerationDevice or any other manufacturer of bone screws.

There is also described a novel and improved method for implantation aswell as embodiments of a positioning device 199 as shown in FIGS. 15-16, an alignment plate 229 as shown in FIGS. 17-21 , a drill template 259,as shown in FIGS. 22-26 and a milling guide and system, as shown inFIGS. 29-36 for the installation of the implants. Broadly, a medial orlateral epicondyle or malleolus osteotomy is performed, the central axisof rotation of the subject joint is established, the subject limb isstabilized in a positioning device, drilling and milling are completedon the subject joint, the aggregate prosthesis is implanted into thejoint cavity, the set plate 47′ is removed and the medial epicondyle isreattached.

The implant 11′, instruments and method are useful in the treatment ofdegenerative joint disease in mammals and allow for a minimally invasiveimplantation technique. The joint is not luxated during the process andthe ligaments and muscles remain attached to the bony structure. Theaggregate prosthesis is implanted in one stage as opposed to separatestage procedures. Prior art involves securing a multi-piece implant inconsecutive steps to the opposing or complementary bone structures, forexample with the elbow, the humerus, radius and ulna.

In one method, offered by way of example only, a radiographic evaluationincluding radiographs as well as arthroscopic surgery are performed onthe subject to determine the degree of disease and to measure andestimate the proper size of implant to be used in the procedure. Atemplate or digital overlay, not shown, is also used to determine thesize of the implant necessary. The subject joint is prepared and amedial joint is exposed for osteotomy. An osteotomy guide (not shown) isclamped to the medial condyle of the subject. The osteotomy guide is ahemostat-like instrument that uses specific anatomical references toperform an accurate resection of the medial epicondyle. A saw blade, notshown, is inserted through a cutting slot and accurately cuts the bone.Once the medial epicondyle has been osteotomized, not shown, the cutportion which is the condylar crown is reflected back along with theattached flexor muscles and medial collateral ligaments, exposing thedistal medial humeral condyle.

The next step involves locating the central axis of rotation of thesubject joint. Using a guide, a 2.5 mm “C.O.R.” (Center of Rotation)bore is drilled through the central axis of rotation of the joint usinga hand drill and a specifically designed instrument, not shown, thataids in location of the central axis of rotation of a hinge joint. TheCOR bore functions as a reference for every surgical step thereafter andaids in proper positioning of the implant as well as positioning of thealignment plate 229, drill template 259 and milling arm 301. In thisinstance a drill is used to drill through the central axis of rotation.Once the COR bore is drilled, a CORpin 70 is inserted medially up to theflange 73 which leaves an extended portion, the upper post 71, extendingout of the medial side for attaching various instruments such as, butnot limited to, the alignment plate, drill guide and milling arm. TheCOR pin 70 extends through the joint and out the lateral side where itis inserted into a COR base 213 on the positioning device 199 and lockedinto position with a thumb screw 214 located along the COR base 213.Broadly, the positioning device 199 as shown in FIGS. 15 and 16stabilizes and supports the joint for ease of operation. The positioningdevice 199 consists of a base arm 201 that runs parallel to an upper arm203, both secured respectively in parallel by adjustment members 205 and207. The base arm 201 and upper arm 203 are secured to a table T asshown in FIG. 15 . A lower support member 209 extends from the lowerbase arm 201 providing support for the COR base 213. The COR base 213has a circular platform support 210 that includes a COR post opening 231and an adjustable arm 215 extending transversely of the COR base 213,the arm 215 having spaced upstanding dual post members 219,219′ that areprovided for limb support. The lower support member 209 is adjustablewith linear clamp 217 to accommodate a variety of subject sizes, and thearm 215 is also adjustable by utilizing a telescoping adjustment member221. Once the COR pin 70 is inserted into COR post recess 231 andsecured into place, the subject limb and joint are positioned in properalignment for the next surgical steps.

For example, a limb may be placed between and cradled with dual postmembers 219, 219′, exposing a medial or lateral joint for placement ofthe COR post 70 through the COR bore in the joint from a medial tolateral aspect and locked into place within the COR base 213 with thethumb screw 214. Once the COR pin 70 is locked in place, the alignmentplate 229 slides over the COR post 71 via opening 261 as shown in FIGS.17 and 18 . The alignment plate 229 broadly is of pentagonalconfiguration with level upper and lower surfaces as shown in FIGS. 19and 21 . The plate 229 is of uniform width with the exception of acentrally located cut-away portion or milling window 230 approximatingthe cross-sectional configuration of the aggregate prosthesis 10′ atapproximately 90° of articulation as shown in FIG. 20 . The millingwindow 230 is defined by lateral edges 234 surrounding the window 230.The configuration of the alignment plate 229 and therefore the millingwindow 230 will vary depending upon the articulation surface to beresected and the configuration of the prosthesis to be inserted. Uponmounting the alignment plate 229 over the COR post 71, the plate 229 isrotated until the cranial edge of the milling window 230 aligns with thecranial osteochondral junction of the humerus. A Steinmann pin ork-wire, 247, shown in FIG. 17 , is inserted through openings 271,273 or275 in the alignment plate 229 and into the distal end of the proximalbone, in this case the humerus. The joint is then brought into a degreeof flexion, and the cranial peg portion 267′ that is part of the millingwindow 230 aligns with the middle of the radius 51′ at which pointcancellous bone screws 235,237, and 239 are inserted through respectiveholes 241,243 and 245 in the alignment plate and into bone,superimposing the plate onto the side of the joint, and locking theplate in the desired position. A second or third k-wire, Steinmann pin,or bone screw may be placed through openings 271,273 or 275 to increasethe stability of the joint. Optionally, a posterior retraction plate253, as shown in FIGS. 27 and 28 , may be used with posterior tabs 255and 257 aiding in retraction of ulnar nerves and soft tissue fromencroaching on the drilling and milling processes. Shoulder edgesupports 254 and 256, as shown in FIG. 20 , are present on the alignmentplate to provide a means for locking the retraction plate 253 inposition. Once the alignment plate 229 is secured to the surface of themedial condyle, a drill guide template 259 is then placed over the CORpost 71, through opening 261 and superimposed on the alignment plate229, as shown in FIGS. 22 and 26 .

The drill template 259 is slightly smaller in overall size than thealignment plate 229. The drill template is of substantially triangularconfiguration having protruding annular bosses extending upwardly anddownwardly from a base 264 of the drill template as shown in FIG. 24 .The template has two annular bosses 263 and 265 that are either 2.7 mmor 3.2 mm in diameter and two additional annular bosses 267 and 269 thatare either 3.2 mm or 4.5 mm in diameter depending upon the size of thejoint. The radial distance of the annular bosses 263, 265, 267 and 269changes with the size of the joint and are designed to allow for a 2.7mm, 3.2 mm, or 4.5 mm drill bit to pass through. Bore 261′ of the drilltemplate slides over the COR post 71 and the annular bosses 263,265,267and 269 align over the appropriate and corresponding openings 263′,265′, 267′ and 269′ in the milling window 230 of the alignment plate229. In use, the drill template 259 is superimposed on the alignmentplate 229 and the four protruding annular bosses 263,265,267, and 269correspond to the cranial and caudal peg bores 263′, 265′, 267′ and 269′on the alignment plate and correspond as well to the humeral implantretaining members 40′, 40″ and the radioulnar implant retaining members23′, 23″. Once the drill template is attached and properly positionedusing the COR post as a reference for the central axis of rotation,drill bits are inserted through the four annular bosses in order to forma portion of the recess for the joint prosthesis. The drill template 259is then removed and the milling process is started.

A specially designed milling apparatus 301 shown in FIGS. 29-36 isutilized to accurately and reproducibly resect a joint and morespecifically to remove articular surfaces and prepare the joint cavityfor insertion of the aggregate prosthesis. The milling system accuratelyremoves remaining cartilage and minimal subcondylar bone whilepreserving good trabecular structure. Subsidence typically occurs whenthe compressive stresses exceed the strength of the bone, resulting inmicrofractures and resorption of trabeculae. Resurfacing of thearticular surfaces of the humerus, the radius and ulna of the canineelbow, for example, is performed with the milling arm 301 and custom endmill 329. The milling arm 301 is adaptable to accommodate various typesof instruments to aid in milling, for example, to prepare the joint forinsertion of the prosthesis and comprises a milling shaft 304, as shownin FIG. 30 , that is designed to be placed over the COR post 71 forstabilization and orientation. The milling arm 301 may take a number offorms but by way of example is shown in FIGS. 32 through 36 as having alinear shaft portion 304 with upwardly extending end mounts 325 and 327as well as adaptors 311 and 313 journaled at opposite ends. Each adaptoris roughly at an acute angle to the shaft 304 and extends downwardlyfrom the end mounts 325 and 327. Each of the end mounts 325 and 327 aredesigned to mill a size specific mill path to accommodate differentsized implants. The adaptors 311 and 313 include sized pin fittings 315and 317 for extension into the alignment plate 229. The opposite endadaptors 311 and 313 allow the milling apparatus to be adaptable for usewith different sizes of implants. For example but not by way oflimitation, once the COR post 71 is inserted in bore 309 or 310, one ofthe adaptor fittings 315 or 317 is inserted into the bore 323 in thealignment plate 229, as shown in FIG. 23 . The placement of adaptorfitting 315 or 317 into the bore 323 in the alignment plate 229, allowsthe milling arm 301 to rotate a pre-determined amount in an arc-shapedconfiguration for accurate removal of designated joint surfaces via theend mill for placement of the prosthesis The opposing ends 305 and 307of the milling arm 301 are utilized depending upon the size ofprosthesis required for implantation. Each size of prosthesis requires acorresponding alignment plate to assure proper and accurate removal ofjoint surfaces.

The custom bone end mill 329 is inserted through entry port 324 or 328and inserted into a rotary handpiece while mill end member 331 passesthrough the milling window 230 of the alignment plate 229 therebyallowing the end mill 329 to pivot, in a controlled fashion, with themilling arm 301 in a radial arc around the center of rotation (i.e. theCOR Post) and simultaneously resurface the joint surfaces forimplantation of the aggregate prosthesis. A depth measuring device ordepth limiting stop, not shown, is used to ensure proper depthpenetration. The mill end member 331 moves through the large arc-shapedmilling window 232 within the alignment plate 229, resulting insimultaneous removal of both the humeral and radioulnar articularsurfaces from a medial or lateral aspect. The milling device 301 enablesa user to lock the milling device over the COR post 70 and rotate themilling device horizontally through a designated arc based on the arcformed by the insertion of the adaptor fittings 315 or 317 and the bore323 as described above. The end mill 329 is inserted through the millingarm bore 324 or 328 and is adapted to pivot around the COR, bearingmembers 341, 343, 347 and an additional bearing member that is notshown, allow the end mill 329 to spin at high speeds via the rotaryhandpiece and motor. The drilling and milling systems allow the surgeonto prepare all pertinent articular surfaces simultaneously and allsurfaces reference the natural central axis of rotation of the joint.The milling window 232 in the alignment plate is configured such thatwhen the adaptor fittings 315 or 317 are mounted on the adaptor in oneof the sockets, the mill end member 331 will remove only the articularsurfaces within range of the guide path and the mill end member cannottravel beyond the path configuration defined by the alignment plate asshown in FIGS. 29 and 30 . Further, the adaptors and adaptor fittingsmay be modified as well as the configuration on the alignment plate toallow for the desired amount of joint surface removal.

An upper portion 326 of the end mill is locked in a rotary handpiecewhich is attached to a low speed high torque motor manufactured byForedom Power Tools and is held in one hand while the milling arm 301 isheld in the other hand and pivoted around the COR pin as the jointsurfaces are milled. This results in a controlled, stable process forremoving articular surfaces and allowing for accurate and easy removalof the articulating joint surfaces, virtually avoiding the potential foroperator error. The milling arm bore 324 and 328 may take many forms butare designed to enable an approach that is parallel to the central axisof rotation of the joint, providing stabilization. The milling window230 as well may take different forms and may accommodate different toolsto allow for varied forms of cartilage and bone removal.

Once the surfaces are simultaneously removed, the alignment guide isleft in place and the aggregate prosthesis is press-fit through thewindow 230 in the alignment plate and into the space removed through themilling and drilling processes.

Due to the insertion of the implant from the medial or lateral aspect,the humeral and radioulnar articulating surfaces may be resurfacedwithout having to luxate or otherwise open or expose the articulatingsurfaces of the joint which could result in unwanted damage to thesurrounding soft tissue. The removal of articular cartilage as well as aminimal amount of subcondylar bone on both sides of the jointsimultaneously without having to disarticulate the joint allows for aminimally invasive procedure.

Referring to FIGS. 2 and 18 , the implant 11′ is lined up with theimplant retaining plate 47′ in place, all four bores lining up with thefour horizontal members 40′, 40″, 23′, 23″ located on the first bonefixation portion 14′ of the humeral component 13′ and the second bonefixation portion 33′ of the radioulnar component 29′. This allows theaggregate implant 10′ to be inserted where the articulating surfaceshave been removed. Using a hammer or press device, not shown, theimplant 10′ will be tapped or pressed into place within the elbow joint.With the members 40′, 40″, 23′, 23″ running horizontally, the implantmay not rotate on a sagittal plane while inside the elbow. Thehorizontal members also prevent the implant from sliding side to sidebased on a press-fit of the joint.

In the case of a canine elbow joint, the implant retaining members 40′,40″, 23′, 23″ are aligned with the four drill bores and are impacted orpounded into the joint cavity so that there is almost no distancebetween the implant and the bone. Primary fixation occurs due to thepress-fit nature of the implant as a result of the retaining members,providing initial stabilization for bone ingrowth. Secondary fixationoccurs as a result of the porous members allowing for bony ingrowthbetween the implant and the existing bony structures. Cementlessfixation is utilized in our method but is set forth as an example, notas a limitation. Once the implant is in place, the implant retainingplate 47′ is removed and the medial epicondylar crown, including theattached ligaments and muscles, is reattached, not shown, using a 3.5 mmcancellous screw, not shown. The cancellous screw is manufactured byVeterinary Orthopedic Implant, Synthes or New Generation Devices.

While a number of exemplary aspects, embodiments and methods have beendiscussed above, those of skill in the art will recognize certainmodifications, permutations, additions and subcombinations thereof. Itis therefore intended that the following appended claims and claimshereafter introduced are interpreted to include all such modifications,permutations, additions and sub-combinations as are within their truespirit and scope.

We claim:
 1. A joint prosthesis system comprising: an implant; analignment guide comprising: a post member extending through a centralaxis of rotation in a prosthesis; and a drill guide member having firstlimiting member for limiting articular surface removal from a joint, thefirst limiting member including a recessed guide for positioning theimplant.
 2. The joint prosthesis system of claim 1, further comprising asecond limiting member, the second limiting member including a drillguide member removably positioned on the post member.
 3. The jointprosthesis system of claim 1, wherein the post member includes aretraction member.
 4. The joint prosthesis system of claim 1, whereinthe post member includes means for removing articular surfaces from thejoint.
 5. The joint prosthesis system of claim 1, wherein the alignmentguide comprising a base member with level upper and lower surfaces, acentrally-located recess portion, and a center of rotation postreceiving member.
 6. The joint prosthesis system of claim 5, wherein thebase member has a substantially pentagonal configuration.
 7. The jointprosthesis system of claim 5, wherein the recess portion is adapted toreceive an aggregate prosthesis therein.
 8. The joint prosthesis systemof claim 5, wherein the base member further includes a posteriorretraction plate.
 9. The joint prosthesis system of claim 8, wherein thebase member includes a shoulder edge support member for locking theposterior retraction plate in position.
 10. The joint prosthesis systemof claim 5, wherein the drill guide member comprising a substantiallytriangular shaped base member having protruding annular bosses extendingupwardly and downwardly from the base member and a center of rotationpost receiving bore.
 11. The joint prosthesis system of claim 10,wherein the annular bosses are sized to accommodate a cutting tooltherethrough.
 12. The joint prosthesis system of claim 11, wherein theannular bosses correspond to cranial and caudal retaining members on anaggregate prosthesis.
 13. An apparatus for removing joint components,comprising; an elongated shaft having opposing journaled end membersadapted to accommodate a center of rotation post; and a pair of openingsfor accommodating a tooling member.
 14. The apparatus of claim 13,wherein the elongated shaft has a mid-section opening and the opposingend members include a joining means for joining the apparatus with analignment plate.
 15. The apparatus of claim 14, wherein the end membersare at an acute angle to the elongated shaft.
 16. The apparatus of claim14, wherein the joining means include pin adaptors of variable size. 17.The apparatus of claim 14, wherein the pair of openings include bearingmembers.
 18. The apparatus of claim 13, further comprising a millingapparatus comprising a milling shaft with adaptors journaled at oppositeends, wherein the adaptors have variable sized pin fittings and areconfigured to receive a center of rotation post.
 19. The apparatus ofclaim 18, wherein the adaptors are at an acute angle and extenddownwardly from the milling shaft.
 20. The apparatus of claim 18,further comprising a mill end sized to pass through an alignment plateand remove limited articular surfaces.